Thin film electroluminescent (TFEL) displays are known and are typically fabricated on glass substrates. Electroluminescent displays with thin film phosphors employing thick film dielectric layers fabricated on ceramic substrates, as exemplified by U.S. Pat. No. 5,432,015 provide greater luminance and superior reliability.
A high luminosity full colour electroluminescent display requires the use of red, green and blue sub-pixels. Optical filters are needed to achieve the required colour coordinates for each sub-pixel. Consequently, the thin film phosphor materials used for each sub-pixel must be patterned so that there is minimal attenuation of the emission spectrum for each colour of pixel by the optical filters. For relatively low-resolution displays, the required patterning can be achieved by depositing the phosphor materials through a shadow mask. For displays with high resolution, however, the shadow mask technique does not provide adequate accuracy, and photolithographic methods must be employed. Photolithographic techniques require the deposition of photoresist films and the etching or lift-off of portions of the phosphor film to provide the required pattern.
Deposition and removal of photoresist films and etching or lift-off of phosphor films typically require the use of solvent solutions that contain water or other protic solvents. Some phosphor materials, for example strontium sulphide are susceptible to hydrolysis, and water and aprotic solvents may degrade the properties of the phosphor materials.
The deficiencies in phosphor materials are most severe with the phosphors used for blue sub-pixels, and may be compensated for to some extent by increasing the area of the blue sub-pixels relative to the area of the red and green sub-pixels. However, such a design modification demands increased performance from the phosphor materials used for the red and green phosphor materials, and requires the use of higher display operating voltages. The higher operating voltages increase the power consumption of the display, decrease the reliability and increase the cost of operating the electronics of the display.
Thick film dielectric structures provide superior resistance to dielectric breakdown, as well as a reduced operating voltage. When deposited on a ceramic substrate, the thick film dielectric structure will withstand higher processing temperatures than TFEL devices on glass substrates. The increased tolerance to higher temperatures facilitates annealing of the phosphor films at higher temperatures, to improve luminosity. However, even with the enhanced luminosity that is obtained, thick film electroluminescent displays have not achieved the phosphor luminance and colour coordinates needed to be fully competitive with cathode ray tube (CRT) displays. Moreover, recent trends in CRT specifications are to higher luminance and higher colour temperature.
Traditionally, cerium-activated strontium sulphide has been the phosphor material of choice for blue electroluminescence. This material has a relatively high efficiency of conversion of electrical to optical energy, of up to about 1 lumen per watt of input power. However, the emission spectrum of cerium-activated strontium sulphide contains a substantial green emission in addition to the required blue emission, producing a cyan colour. This necessitates the use of an optical filter to achieve acceptable blue colour coordinates. The filter substantially attenuates the luminosity of the phosphor, and it is therefore difficult to achieve adequate display luminosity. It is known that the spectral emission of cerium-activated strontium sulphide phosphor may be shifted to some degree towards blue by controlling deposition conditions and activator concentration, but not to an extent required to eliminate the need for an optical filter.
Alternate blue phosphor materials have been evaluated. These include cerium-activated alkaline earth thiogallate compounds, which give good blue colour coordinates, but have relatively poor luminosity and stability. Lead-activated calcium sulphide has also been shown to provide excellent blue colour coordinates when the lead activator is introduced as a dimer, but this material is subject to degradation of the dimer species into isolated activator atoms that provide an ultraviolet rather than blue emission. Europium-activated barium thioaluminate provides excellent blue colour coordinates and higher luminance, but must be annealed at high temperature to achieve this performance.
Improvements in the luminance and emission spectrum of phosphor materials used for blue sub-pixels in full colour AC electroluminescent displays employing thick film dielectric layers with a high dielectric constant would be useful. The thick film dielectric structure would provide superior resistance to dielectric breakdown as well as a reduced operating voltage, compared to thin film electroluminescent (TFEL) displays.